C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), R. Santucci (Universidade Federal de Goi��s), A. J. Castro-Tirado (IAA-CSIC and UMA), I. P��rez-Garc��a, Y.-D. Hu (IAA-CSIC), F. Navarete (IAG-USP, RSS/NOIRLab), and M. Makler (UNSAM/CBPF), on behalf of a larger collaboration, report:
Following the detection of the very bright GRB 230307A (Burns et al. GCNC 33414) by Fermi/GBM (the Fermi GBM team, GCNC 33405; Dalessi, GCNC 33407; Dalessi and Roberts, GCNC 33411), GECAM (Xiong et al., GCNC 33406), Solar Orbiter/STIX (Xiao and Krucker, GCNC 33410), AGILE/MCAL (Casentini et al., GCNC. 33412), INTEGRAL/SPI-ACS, Mars-Odyssey/HEND (Kozyrev et al. GCNC 33413), AstroSat/CZTI (Navaneeth et al. GCNC 33415), GRBAlpha (Dafcikova et al. GCNC 33418), VZLUSAT-2 (Ripa et al. GCNC 33424), Konus-WIND (Svinkin et al. GCNC 33427) Swift-BAT (Tohuvavohu, GCNC 33431) and ASO-S/HXI (Li et al. GCNC 33438), we report z-band imaging and optical spectroscopy of the GRB 230307A from 9 and 10 March 2023 with the Goodman high-throughput imaging spectrograph on the SOAR 4.1m telescope at Cerro Pach��n, Chile. The z-band imaging consisted of a cumulative 310s of exposure starting at 2023-03-10T00:13:45 (MJD 60013.0095, 56.5 hr from burst) and 600s starting at 2023-03-11T00:19:08 (MJD 60014.0133, 80.5 hr from burst) at the site of the reported optical counterpart to GRB 230307A (Levan et al. GCNC 33439, Lipunov et al. GCN 33441, Fulton et al. GCNC 33443, O'Connor et al. GCNC 33447, Im et al. GCNC 33449, Vanderspek et al. GCNC 33453). Using forced photometry at the site of the counterpart (RA=04:03:25.83, Dec=-75:22:42.7), we obtained a marginal detection in both epochs of z=21.8+/-0.3 AB mag. The lack of any change between the two epochs suggests we may be seeing a faint background source consistent with the host galaxy of GRB 230307A.
We also obtained spectroscopy at the site of the counterpart at 2023-03-10T00:28:17 with the 400 l/mm grating on SOAR/Goodman covering a wavelength range of 3000-7000 angstroms for a cumulative exposure time of 1500s. We oriented the 1.0" Goodman long slit to simultaneously observe the optical afterglow of GRB 230307A and the red source roughly 3 arcseconds to the southwest reported in Levan et al. (GCNC 33439) as a possible host galaxy. There is no signature of emission from the GRB afterglow in our spectra, but we detect red continuum emission from 6000-7000 angstroms consistent with the r=20.8 mag, i=19.5 mag, z=18.9 mag (Legacy Survey DR10; Dey et al., 2019, AJ, 157, 168) to the southwest. Combined with the fact that this source is unresolved in our imaging and has a significant parallax in the Gaia DR3 catalog (Gaia Collaboration et al., 2022), we consider this source to be a foreground star as pointed out by Lipunov et al. (GCNC 33441) and not the host galaxy of GRB 230307A.
Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Minist��rio da Ci��ncia, Tecnologia e Inova����es do Brasil (MCTI/LNA), the US National Science Foundation���s NOIRLab, the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).